No difference, really. Both can be used in nearly all situations, and everyone will understand you. However, "when will you return?" Is more familiar and commonly used. "When will you be returning?" Sounds more formal and is less commonly used.

2. I take for granted the person is returning at some point in time. I want more information about when exactly. 1. A broader question. I may or may not know if the person has plans to return or not. I want to know when.